[Diy_efi] CMOS Levels

[bcroe at juno.com]

Carter Shore gave very good advice.  If you are considering
using a TTL gate output to drive CMOS, try an appropriate 
pullup resistor to raise the "high" close to 5 V; this usually 
works but not 100%.  NEVER drive a 50% CMOS input 
with straight TTL.  

Gate input "thresholds" are made as narrow a band as 
possible (for stability, except hysteresis gates), no gate 
input would ever have a threshold band of 10% to 70%.  

18 Jun 03 Jeffrey Engel <jengeltx at yahoo.com> writes:
> Bruce,
> 
> We're getting off into the weeds here.  The original
> RCA 1800-series chips did have an odd set of logic
> levels.  I believe Larry Ing's assertion that the
> thresholds were 10 and 70 percent is correct for the
> chips in question.
> 
> The HC and the HCT series chips have the thesholds you
> describe.
> 
> I suggested using HC or HCT logic to drive the 1843
> because TTL doesn't (via specs) meet the higher '1'
> threshold the 1800-series CMOS needs, while the HC or
> HCT has rail-to-rail output and meets those specs
> easily.  It turns out that lightly-loaded TTL _will_
> meet the higher '1' threshold, so my suggestion turned
> out to be more superstition than fact.  Sorry.
> 
> Jeff Engel
> --- bcroe at juno.com wrote:
> > That is not right.  The threshold is not 10 and 70%,
> > although 
> > the OUTPUT drive of some chips might be.  The
> > threshold 
> > depends on the type chip.  An HC chip INPUT
> > threshold is 
> > close to 50%, but an HCT chip expects a minimum
> > swing 
> > of 0.8V to 2V.  Check the specific type to be sure.
> > 
> > Bruce Roe

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